[1.7.x] Removed doc reference to removed HttpResponse.mimetype.

Backport of da009ce32c from master
[1.7.x] Corrected content_type parameter name in sitemap docs.

mimetype was deprecated in 1.5 per 11ec0253ab4cc926ab9e77619132cb398231ac33.

Backport of a7443c24a3 from master
[1.7.x] Fixed #23265 -- Used system-specific encoding in runserver

Thanks SpaceFox for the report.
Backport of 055d95fce066 from master.
[1.7.x] Fixed #23291 -- Don't add Unicode varname/value to environment.

Windows doesn't like that and it causes 114 failures in the
admin_scripts tests with Python 2.x.

0bfb53866199f366ed140d49938fd185e5898156 from master.
[1.7.x] Fixed a grammar error in docs/ref/forms/api.txt.

Backport of bef5b9b7cd from master